1. Field of the Invention
This invention relates generally to handling devices for a flexible sheet material and more particularly to a novel gripping device for such sheet material.
2. Prior Art
As will appear from the ensuing description, the sheet gripping device of the invention may be utilized for a variety of sheet gripping applications. However, the gripping device is intended primarily for use as a screen anchor bar for a silk screen printing frame, or screen chase as it is referred to in the silk screen printing industry. The invention will be described in connection with this particular application.
Simply stated, a screen chase comprises a rectangular frame having a central opening bounded by four frame members forming the four sides of the frame, and a printing screen overlying the frame and secured along its edges to screen anchor bars carried by the frame members. These anchor bars include gripping means for gripping the screen edges and are adjustable laterally relative to their respective frame members for biaxially stressing the printing screen. The gripping means are releasable to permit removal and replacement of the screen.
The prior art is replete with a vast assortment of such silk screen printing chases. Included in this prior printing chase art are my U.S. Pat. Nos. 3,482,343 and 3,553,862, and the following additional U.S. Pat. Nos. 2,803,086; 2,854,922; 2,893,162; 3,422,554, Belgian Pat. No. 519,040.
This invention is concerned with improving the anchor bar screen gripping means of silk screen printing chases of the character described, particularly those which utilize cylindrical screen gripper bars or rods such as described in my U.S. Pat. No. 3,553,862. This latter type of gripping means comprises cylindrical gripper rod means, which may be a single gripper rod or multiple rods as described in my patent, contained within a longitudinal channel in an anchor bar. The printing screen is inserted through the channel and around the gripper rod means in such a way that tension in the screen draws the rod means to a gripping position in the channel wherein the rod means is disposed in wedging relation between the channel side walls and the screen is gripped between the side walls and the rod means. While this type of gripping means is suitable for many printing chase applications, it has the disadvantage that the screen tension, which acts tangentially of the gripper rod means, tends to rotate the latter and thereby cause release of the screen tension. Other types of existing screen gripping means tend to be complicated, costly, and/or difficult to manipulate to effect gripping of a screen.